The accuracy of spectrometric analyses of objects and materials, using transmission and emission measurements, will depend substantially upon the integrity of the radiation beams detected. Infrared spectrometers currently used for making such measurements may employ a Michelson interferometer, through which is established an optical path between a silicon carbide (globar) internal radiation source and at least one transmission port. The instrument has a separate port for making emission measurements, which lies in optical communication with the transmission port by virtue of a shared portion of the optical path through the interferometer. Both the transmission and the emission characteristics of a sample placed beyond the transmission port can therefore be determined utilizing such a spectrometer.
Despite the fact that the internal instrument radiation source is used only in making transmission measurements, it is highly desirable to maintain it in operation during emission measurements as well, primarily because of the time that is required to bring the element back to a stable temperature condition after it has been allowed to cool. A difficulty arises however because of the tendency for stray or divergent radiation from the globar to be so transmitted and refracted by the beam-splitter of the interferometer as to appear at the emission detector, thereby corrupting the true emission signal and significantly compromising the accuracy of the measurement. Bellar et al U.S. Pat. No. 4,511,986, issued Apr. 16, 1985, provides a method and apparatus for simultaneously recording multiple FT-IR signals using an interferometer, which signals may relate to the temperature, pressure or mass spectrum of a sample; however it does not address the foregoing problem.
Accordingly, it is the broad object of the present invention to provide an optical instrument, adapted for selective use for making transmission and emission measurements, wherein divergent radiation from the internal radiation source beam is blocked sufficiently to avoid substantially the corruption of emission measurement signals.
Other objects of the invention are to provide such an instrument incorporating an interferometer, in which is provided a device having structure disposed in the path of divergent source beam radiation, so as to block passage into the interferometer optics of such deviant radiation.
More specific objects are to provide such an instrument in which the blocking device partially shields a beam-splitter so as to permit the source beam radiation to pass into the interferometer optics through only one portion thereof and within a defined stratum; and to provide such an instrument by which both transmission and emission measurements can be made selectively and sequentially, with the internal radiation source maintained in operation and without substantial corruption of the emission beam.
An additional object of the invention is to provide a blocking device having the foregoing features and advantages, which device may in addition be of relatively simple and inexpensive construction.
A further object is to provide a method for selectively measuring, in rapid succession, radiation emitted from, and radiation transmitted through or reflected by, a sample, without extinguishing the radiation source means used in making the transmission or reflection measurements.